In the philosophy of perception, critical realism is the theory that some of our sense-data (for example, those of primary qualities) can and do accurately represent external objects, properties, and events, while other of our sense-data (for example, those of secondary qualities and perceptual illusions) do not accurately represent any external objects, properties, and events. In short, critical realism refers to any position that maintains that there exists an objectively knowable, mind-independent reality, whilst acknowledging the roles of perception and cognition.

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According to Locke and Descartes, some sense-data, namely the sense-data of secondary qualities, do not represent anything in the external world, even if they are caused by external qualities (primary qualities). Thus it is natural to adopt a theory of critical realism.

By its talk of sense-data and representation, this theory depends on or presupposes the truth of representationalism. If critical realism is correct, then representationalism would have to be a correct theory of perception.

Critical realism is presently most commonly associated with the work of Roy Bhaskar. Bhaskar developed a general philosophy of science that he described as Transcendental Realism, and a special philosophy of the human sciences that he called Critical Naturalism. The two terms were elided by other authors to form the umbrella term Critical Realism.

Transcendental Realism refers to the fact that in order for scientific investigation to take place, the object of that investigation must have real, manipulable, internal mechanisms that can be triggered to produce particular outcomes. This is what we do when we conduct experiments. This stands in contrast to empiricist scientists' claim that all scientists can do is observe the relationship between cause and effect. The implication of this is that science should be understood as an ongoing process in which scientists improve the concepts they use to understand the mechanisms that they study. It should not, in contrast to the claim of empiricists, be about the identification of a coincidence between a postulated 'independent variable' and 'dependent variable'. Positivism/falsification are also rejected due to the observation that it is highly-plausible that a mechanism will exist but either a) go un-activated, b) be activated, but imperceived, c) be activated, but counteracted by other mechanisms, which result in it having unpredictable effects. Thus, non-realisation of a posited mechanism can not (in contrast to the claim of positivists) be taken to signify its non-existence.

Critical Naturalism argues that the Transcendental Realist model of science is equally applicable to both the physical and the human worlds. However, when we study the human world we are studying something fundamentally different from the physical world and must therefore adapt our strategy to studying it. Critical Naturalism therefore prescribes social scientific method which seeks to identify the mechanisms producing social events, but with a recognition that these are in a much greater state of flux than they are in the physical world (as human structures change much more readily than those of, say, a leaf). In particular, we must understand that human agency is made possible by social structures that themselves require the reproduction of certain actions/pre-conditions. Further, the individuals that inhabit these social structures are capable of consciously reflecting upon, and changing, the actions that produce them - a practice that is in part facilitated by social scientific research.

Since Bhaskar made the first big steps in popularising the theory of critical realism in the 1970s, it has become one of the major strands of social scientific method - rivalling positivism/empiricism, and post-structuralism/relativism/interpretivism.

An edited volume, - Critical Realism: Essential Readings - is the best available reader in critical realism.

There is also a Journal of Critical Realism, which publishes articles on the theory and results of the practice of critical realist social science.

Since his development of Critical Realism, Bhaskar has gone on to develop a philosophical system he calls Dialectical Critical Realism, which is most clearly outlined in his weighty book - Dialectic: the pulse of freedom.

Bhaskar is frequently criticised for the density and obscurity of his writing. An accessible introduction was written by Andrew Collier. Andrew Sayer has written accessible texts on critical realism in social science.

Critical realism is a name that a community of scientists turn theologians apply to themselves. They are influenced by the Scientist turned philosopher Michael Polanyi. Polanyi's ideas where taken up enthusiastically by T. F. Torrance whose work in this area has influenced many theologians wishing to call themselves Critical Realists. This community also includes John Polkinghorne, Ian Barbour, and Arthur Peacocke. The aim of the group is to show that the language of science and Christian theology are similar, forming a starting point for a dialogue between the two. Alister McGrath and Wentzel van Huyssteen (the latter of Princeton Theological Seminary) are recent contributors to this strand. Tom Wright, New Testament scholar and AnglicanBishop of Durham also writes on this topic as evidenced by:

...I propose a form of critical realism. This is a way of describing the process of "knowing" that acknowledges the reality of the thing known, as something other than the knower (hence "realism"), while fully acknowledging that the only access we have to this reality lies along the spiralling path of appropriate dialogue or conversation between the knower and the thing known (hence "critical"). (The New Testament and the People of God, pp. 35)